Historically, polyethylene terephthalate has been difficult to mold at molding temperatures below about 110.degree. C. because its crystallization rate is so slow and uneven that warped articles are obtained. Moreover the articles tend to stick to the mold and are difficult to remove. Recently, it has been found that polyethylene terephthalate articles of good quality can be obtained by molding at lower temperatures, e.g., 80.degree.-100.degree. C., if certain materials are added to the resin prior to molding. These additive materials increase the rate of crystallization, and molded articles having smooth glossy surfaces that are easily removable from molds, are obtained when the materials are employed. The additive materials are (1) sodium or potassium salts of a selected hydrocarbon acid or sodium or potassium salts of selected organic polymers containing pendant carboxyl groups and (2) selected low molecular weight organic compounds that are esters, ketones, sulfones, sulfoxides, nitriles or amides.
However, many of these low molecular weight organic compounds are somewhat volatile at temperatures of 80.degree. C. or above, and tend to vaporize and redeposit on the mold surfaces. It would be advantageous to find low molecular weight organic compounds that do not vaporize and redeposit. Moreover, when the low molecular weight organic compound is an ester, many of the esters undergo a reaction called "ester-interchange" with the PET. In the other words, the low molecular weight R and R.sup.1 groups in the ester ##STR1## will react with a long chain group in polyethylene terephthalate (PET) by breaking a bond at a ##STR2## grouping, thus replacing a long chain with a short R-- group. The interchange can be depicted as follows: ##STR3## (R and R.sup.1 =low mol wt. alkyl), thus the net result is to shorten the overall average chain length of the PET in the mixture. This is detrimental and causes a lowering of the strength of plastic articles molded from PET resins.
Hindered low molecular weight esters will not undergo ester interchange. It is difficult for the ##STR4## bond in hindered esters to break because of its protected nature. For example, a hindered ester such as ##STR5## will not undergo ester interchange to any significant extent. However, such esters, because of their rather symetrical shape due to their "hindered" nature, are more volatile than their straight chain ester counterparts and are subject to the volatility deficiencies described above.
It would be beneficial to find an ester that exhibited low volatility while at the same time did not tend to undergo ester interchange when present with polyethylene terephthalate in a molding system.